On world editor you will always render 9 quadrants (like in game) moving around the map will load the others too.
Splitting a map is not so easy, because the objects are saved on the map coordinates, so:
Start Object000
25827.449219 -14599.858398 16913.000000 -- x y z
18625549 Propery id
0.000000#0.000000#0.000000
0.000000
End Object
You have a map 25x25 for example, and you have this object in the center of map (example folder number 009001, in areadata.txt)
You will have to go and recalculate the position by removing the number of spaces of each folder before it. Otherwise it will set it outside the map in the same position as if it were still a 25x25 map
You have to calculate this thing for each x and y, in any case it becomes a lot of work, maybe with chatgpt you can automate the work, I've never tried.
You're better off using the 25x25 map if you want to keep the "open world" structure or re-arrange the objects manually.
Technically (I've been using a single, gigantic, detailed map for years on my project) you don't have any lag or other problems. The only "problem" is related to the regens, which are classic regens full of monsters and are not very suitable.